Magnetic field detection using magnetohydrodynamic effect

ABSTRACT

An IMD may transition to an MRI mode automatically in response to detecting one or more conditions indicative of the presence of a strong magnetic field. Large static magnetic fields, such as those produced by an MRI device, may interact with the blood of a patient as it flows through the magnetic field to produce a voltage, a phenomenon referred to as the magnetohydrodynamic (MHD) effect. The voltage produced by the MHD effect is proportional to the strength of the magnetic field. As such, the voltage produced by blood flow in the strong magnetic field of an MRI device may result in a change in a characteristic of an electrogram (EGM). The IMD may detect the change in the characteristic of the EGM caused by the MHD effect and transition to operation in the MRI mode in response to at least the change in the EGM.

TECHNICAL FIELD

This disclosure relates generally to implantable medical systems. Inparticular, this disclosure describes techniques for detecting magneticresonance imaging (MRI) devices using the magnetohydrodynamic effect.

BACKGROUND

A wide variety of implantable medical systems that deliver a therapy ormonitor a physiologic condition of a patient have been clinicallyimplanted or proposed for clinical implantation in patients. Theimplantable medical system may include an implantable medical leadconnected to an implantable medical device (IMD). For example,implantable leads are commonly connected to implantable pacemakers,defibrillators, cardioverters, or the like, to form an implantablecardiac system that provides electrical stimulation to the heart orsensing of electrical activity of the heart. The electrical stimulationpulses can be delivered to the heart and the sensed electrical signalscan be sensed by electrodes disposed on the leads, e.g., typically neardistal ends of the leads. Implantable leads are also used inneurological devices, muscular stimulation therapy, gastric systemstimulators and other implantable medical devices (IMDs).

Patients that have implantable medical systems may benefit, or evenrequire, various medical imaging procedures to obtain images of internalstructures of the patient. One common medical imaging procedure ismagnetic resonance imaging (MRI). MRI procedures may generate higherresolution and/or better contrast images (particularly of soft tissues)than other medical imaging techniques. MRI procedures also generatethese images without delivering ionizing radiation to the body of thepatient, and, as a result, MRI procedures may be repeated withoutexposing the patient to such radiation.

During an MRI procedure, the patient or a particular part of thepatient's body is positioned within an MRI device. The MRI devicegenerates a variety of magnetic and electromagnetic fields to obtain theimages of the patient, including a static magnetic field, gradientmagnetic fields, and radio frequency (RF) fields. The static magneticfield may be generated by a primary magnet within the MRI device and maybe present prior to initiation of the MRI procedure. The gradientmagnetic fields may be generated by electromagnets of the MRI device andmay be present during the MRI procedure. The RF fields may be generatedby transmitting/receiving coils of the MRI device and may be presentduring the MRI procedure. If the patient undergoing the MRI procedurehas an implantable medical system, the various fields produced by theMRI device may have an effect on the operation of the medical leadsand/or the IMD to which the leads are coupled. For example, the gradientmagnetic fields or the RF fields generated during the MRI procedure mayinduce energy on the implantable leads (e.g., in the form of a current),which may cause oversensing by the IMD.

SUMMARY

The IMD may transition from operation in a normal mode to another modein the presence of a magnetic field to reduce the likelihood ofinterference. The IMD may transition from the normal mode to an MRI modein response to detecting presence of a large static magnetic fieldassociated with an MRI device. As described in detail herein, the IMDmay detect the presence of the magnetic field by analyzing anelectrogram (EGM). Blood flow through a magnetic field may produce avoltage, a phenomenon referred to as the magnetohydrodynamic (MHD)effect. The voltage produced by the MHD effect has a linear relationshipwith the strength of the magnetic field. As such, a strong magneticfield may result in a change in a characteristic of the EGM, such as achange in a T-wave or an S-T segment of the EGM. Upon identifying such achange, the IMD may transition to the MRI mode.

In addition to transitioning from the normal mode to the MRI mode inresponse to detecting a strong magnetic field, the IMD may transitionfrom the normal mode to a “magnet mode” in response to detectingmagnetic fields of a smaller strength. For example, the IMD maytransition to the magnet mode in response to detecting the presence of amagnetic field of a handheld magnet, such as a telemetry head magnet ora patient magnet. The strength of such a magnetic field is typicallymuch smaller than the strength of the magnetic fields associated withMRI devices. As such, the handheld magnet typically does not producemuch of an MHD effect. The IMD may therefore use the change in the EGMcaused by the MHD effect to differentiate a magnetic field generated bya handheld magnet or other smaller strength magnet from a staticmagnetic field of an MRI device.

For example, the IMD may have a single threshold magnetic field sensorthat determines the presence of a magnetic field having a strength thatexceeds a threshold (e.g., 1 mT). In this case, IMD 22 is unable todetermine whether the magnetic field is associated with an MRI device orother magnet, such as the handheld magnet. However, the IMD monitors fora change in the EGM due the MHD effect and transitions to the magnetmode when no change in the EGM is identified and transitions to the MRImode when the change in the EGM is identified. In this manner, thechange in the EGM due to the MHD effect may be used to differentiatemagnetic fields of different strengths.

In one example, this disclosure is directed to a method comprisingdetecting presence of a magnetic field based on the output of a magneticfield sensor, monitoring an electrogram (EGM) measured by an implantablemedical device in response to detecting the presence of the magneticfield, identifying a change in a characteristic of the EGM indicative ofthe presence of the magnetic field, and adjusting operation of theimplantable medical device based on the output of the magnetic fieldsensor and the change in the characteristic of the EGM.

In another example, this disclosure is directed to an implantablemedical system comprising at least one implantable medical lead thatincludes at least one electrode and an implantable medical deviceconnected to the medical lead. The implantable medical device includes amagnetic field sensor and a control module configured to detect presenceof a magnetic field based on the output of the magnetic field sensor,monitor an electrogram (EGM) measured by an implantable medical devicein response to detecting the presence of the magnetic field, identify achange in a characteristic of the EGM indicative of the presence of themagnetic field, and adjust operation of the implantable medical devicebased on the output of the magnetic field sensor and the change in thecharacteristic of the EGM.

In a further example, this disclosure is directed to a computer-readablemedium comprising instructions that, when executed, cause an implantablemedical device to detect presence of a magnetic field based on theoutput of a magnetic field sensor, monitor an electrogram (EGM) measuredby an implantable medical device in response to detecting the presenceof the magnetic field, identify a change in a characteristic of the EGMindicative of the presence of the magnetic field, and adjust operationof the implantable medical device based on the output of the magneticfield sensor and the change in the characteristic of the EGM.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the techniques as described in detailwithin the accompanying drawings and description below. Further detailsof one or more examples are set forth in the accompanying drawings andthe description below. Other features, objects, and advantages will beapparent from the description and drawings, and from the statementsprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating an environment in which apatient with an implantable medical system is exposed to externalfields.

FIG. 2 is a conceptual diagram illustrating an example implantablemedical system.

FIG. 3 is a functional block diagram of an example configuration ofelectronic components of an implantable medical device.

FIG. 4 is a flow diagram illustrating example operation of animplantable medical device detecting presence of a magnetic field basedon a change in a characteristic of an EGM signal.

FIG. 5 is a flow diagram illustrating example operation of animplantable medical device distinguishing between different types ofmagnetic fields based on a change in a characteristic of an EGM signal.

FIG. 6 is a flow diagram illustrating another example operation of animplantable medical device distinguishing between different types ofmagnetic fields based on a change in a characteristic of an EGM signal.

FIG. 7 is a flow diagram illustrating another example operation of animplantable medical device distinguishing between different types ofmagnetic fields based on a change in a characteristic of an EGM signal.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram illustrating an environment 10 in which apatient 12 with an implantable medical system 14 is exposed to anexternal field 18. In the example illustrated in FIG. 1, environment 10includes an MRI device 16 that generates external field 18. MRI device16 generates magnetic and RF fields to produce images of body structuresfor diagnosing injuries, diseases and/or disorders. In particular, MRIdevice 16 generates a static magnetic field, gradient magnetic fieldsand RF fields as is well known in the art. The static magnetic field isa large non time-varying magnetic field that is typically always presentaround MRI device 16 whether or not an MRI procedure is in progress.Gradient magnetic fields are pulsed magnetic fields that are typicallyonly present while the MRI procedure is in progress. RF fields arepulsed high frequency fields that are also typically only present whilethe MRI procedure is in progress.

The magnitude, frequency or other characteristic of the static magneticfield, gradient magnetic fields and RF fields may vary based on the typeof MRI device producing the field or the type of MRI procedure beingperformed. A 1.5 T MRI device, for example, will produce a staticmagnetic field of about 1.5 Tesla (T) and have a corresponding RFfrequency of about 64 megahertz (MHz) while a 3.0 T MRI device willproduce a static magnetic field of about 3.0 Tesla and have acorresponding RF frequency of about 128 MHz. However, other MRI devicesmay generate different fields.

Implantable medical system 14 may, in one example, include an IMDconnected to one or more leads. The IMD may be an implantable cardiacdevice that senses electrical activity of a heart of patient 12 and/orprovides electrical stimulation therapy to the heart of patient 12. Forexample, the IMD may be an implantable pacemaker, implantablecardioverter defibrillator (ICD), cardiac resynchronization therapydefibrillator (CRT-D), cardioverter device, or combinations thereof. TheIMD may alternatively be a non-cardiac implantable device, such as animplantable neurostimulator or other device that provides electricalstimulation therapy.

Some or all of the various types of fields produced by MRI device 16(which are represented by external field 18) may have undesirableeffects on implantable medical system 14. In one example, the gradientmagnetic fields and/or the RF fields generated during the MRI proceduremay induce energy on the conductors of the leads (e.g., in the form of acurrent). The induced energy on the leads may be conducted to the IMDand inappropriately detected as physiological signals, a phenomenonoften referred to as oversensing. The detection of the induced energy onthe leads as physiological signals may result in the IMD deliveringtherapy when it is not desired (e.g., triggering a pacing pulse) orwithholding therapy when it is desired (e.g., inhibiting a pacingpulse).

The IMD may be configured to operate in an “MRI mode” upon detecting thepresence of MRI device 16. Operation of the IMD in the “MRI mode” mayrefer to an operating state of the IMD in which the undesirable effects(e.g., oversensing) that may be caused by the gradient magnetic fieldsand RF fields of MRI device 16 are reduced, and possibly eliminated.When operating in the MRI mode, the IMD is configured to operate withdifferent functionality compared to the “normal mode” of operation(which may also be the current mode of operation). In one example, theIMD may operate in either a non-pacing mode (e.g., sensing only mode) orin an asynchronous pacing mode as the MRI mode. The IMD may also turnoff high voltage therapy (e.g., defibrillation therapy) while operatingin the MRI mode. The IMD may also turn off telemetry functionality,e.g., wakeup or other telemetry activity, during operation in the MRImode. Other adjustments may be made as described herein. In this manner,patient 12 having implanted medical system 14 may receive an MRIprocedure that exposes implantable medical system 14 to external fields,such as external field 18 of FIG. 1, with a reduced likelihood ofinterference with operation of the IMD.

The IMD may transition to the MRI mode automatically in response todetecting one or more conditions indicative of the presence of MRIdevice 16. Large static magnetic fields, such as those produced by MRIdevice 16, may interact with the blood of patient 12 as it flows throughthe magnetic field. In particular, the blood flow through the largestatic magnetic field of MRI device 16 may produce a voltage, aphenomenon referred to as the magnetohydrodynamic (MHD) effect. Thevoltages produced by the MHD effect may result in a change in acharacteristic of an electrogram (EGM) detected by the IMD. In thismanner, the change in the characteristic of the EGM is indicative of thepresence of a large static magnetic field. As will be described indetail herein, the IMD may detect the change in the characteristic ofthe EGM due to the MHD effect and transition to operation in the MRImode in response to the detection. In other examples, the IMD maymonitor for the MHD effect in conjunction with other criteria indicativeof a magnetic field.

After the MRI procedure is complete, the IMD may transition back to thenormal mode of operation or the mode of operation immediately prior totransitioning to the MRI mode, e.g., turn high voltage therapy back onand/or have pacing that is triggered and/or inhibited as a function ofsensed signals. The IMD may automatically revert to the normal mode ofoperation in response to no longer detecting the MHD effect on the EGMsignal, no longer detecting another condition indicative of presence ofthe magnetic field, after expiration of a timer or a combinationthereof. Alternatively, the IMD may be manually programmed into thenormal mode of operation via a command received from an external device,such as programming device.

FIG. 2 is a conceptual diagram illustrating an example implantablemedical system 20. Implantable medical system 20 may correspond withimplantable medical system 14 of FIG. 1. Implantable medical system 20includes an IMD 22 connected to leads 24 a,b. IMD 22 includes a housing26 within which electrical components and a power source of IMD 22 arehoused. Housing 26 can be formed from conductive materials,non-conductive materials or a combination thereof. As will be describedin further detail herein, housing 26 may house one or more processors,memories, transmitters, receivers, sensors, sensing circuitry, therapycircuitry and other appropriate components.

Leads 24 a,b each includes one or more electrodes. In the exampleillustrated in FIG. 2, leads 24 a,b each include a respective tipelectrode 28 a,b and ring electrode 30 a,b located near a distal end oftheir respective leads 24 a,b. When implanted, tip electrodes 28 a,band/or ring electrodes 30 a,b are placed relative to or in a selectedtissue, muscle, nerve or other location within the patient 12. In theexample illustrated in FIG. 2, tip electrodes 28 a,b are extendablehelically shaped electrodes to facilitate fixation of the distal end ofleads 24 a,b to the target location within patient 12. In this manner,tip electrodes 28 a,b are formed to define a fixation mechanism. Inother embodiments, one or both of tip electrodes 28 a,b may be formed todefine fixation mechanisms of other structures. In other instances,leads 24 a,b may include a fixation mechanism separate from tipelectrode 28 a,b. Fixation mechanisms can be any appropriate type,including a grapple mechanism, a helical or screw mechanism, adrug-coated connection mechanism in which the drug(s) serves to reduceinfection and/or swelling of the tissue, or other attachment mechanism.

Leads 24 a,b are connected at a proximal end to IMD 22 via connectorblock 32. Connector block 32 may include one or more receptacles thatinterconnect with one or more connector terminals located on theproximal end of leads 24 a,b. Leads 24 a,b are ultimately electricallyconnected to one or more of the electrical components within housing 26.

One or more conductors (not shown in FIG. 2) extend within leads 24 a,bfrom connector block 32 along the length of the lead to engage the ringelectrode 30 a,b and tip electrode 28 a,b, respectively. In this manner,each of tip electrodes 28 a,b and ring electrodes 30 a,b is electricallycoupled to a respective conductor within its associated lead bodies. Forexample, a first electrical conductor can extend along the length of thebody of lead 24 a from connector block 32 and electrically couple to tipelectrode 28 a and a second electrical conductor can extend along thelength of the body of lead 24 a from connector block 32 and electricallycouple to ring electrode 30 a. The respective conductors mayelectrically couple to circuitry, such as a therapy module or a sensingmodule, of IMD 22 via connections in connector block 32. The electricalconductors transmit therapy from the therapy module within IMD 22 to oneor more of electrodes 28 a,b and 30 a,b and transmit sensed electricalsignals from one or more of electrodes 28 a,b and 30 a,b to the sensingmodule within IMD 22.

As will be described in further detail herein, IMD 22 may transitionfrom operation in a “normal mode” to another mode in the presence of amagnetic field. Operation of IMD 22 in the normal mode may describe acurrent operating mode or a typical operating state of the IMD. Thetypical operating state may involve operation of ordinary therapy and/orsensing modes in the IMD to provide optimal therapy to patient 12. Inthe case of an IMD functioning as an implantablecardioverter-defibrillator, for example, the normal mode may permitnormal sensing to support normal pacing, cardioversion and/ordefibrillation therapy functions.

IMD 22 may transition from the normal mode or the current mode to theMRI mode in response to detecting a static magnetic field generated byMRI device 16. As described in detail herein, IMD 22 may detect a changein the characteristic of the EGM due to the MHD effect and adjustoperation of IMD 22 in response to the detection. IMD 22 may monitor forthe MHD effect in conjunction with other factors indicative of amagnetic field to detect presence of MRI device 16.

In addition to transitioning from the normal mode to the MRI mode inresponse to detecting a strong magnetic field, the IMD may transitionfrom the normal mode to a “magnet mode” in response to detectingmagnetic fields of a smaller strength than the magnetic fieldsassociated with MRI device 16. In the magnet mode, IMD 22 may operatewith different functionality than the normal mode and the MRI mode. IMD22 may adjust therapy or sensing operations of IMD 22 during operationin the magnet mode, such as transitioning to an asynchronous pacing modeand/or turning off high voltage therapy (e.g., defibrillation therapy).The magnet mode is typically permanently configured by the manufacturer.In other words, the operational parameters of the magnet mode (e.g., thepacing mode, pacing rate, pacing pulse amplitude or othercharacteristic) are not configurable by a physician, technician,clinician or other user. To the contrary, the MRI mode is configurablesuch that it may be adjusted on a patient-by-patient basis. IMD 22 mayalso, in some instances, activate telemetry circuitry within IMD 22 toinitiate communication, e.g., transfer of data, between IMD 22 andexternal device 34. IMD 22 may, for example, wake up or otherwise poweron the telemetry circuitry of IMD 22 to monitor for a communication fromexternal device 34 or transmit a communication to external device 34 inresponse to detecting the small magnetic field of the handheld magnet.

As indicated above, IMD 22 may transition to different operating modesin response to detecting different magnetic fields, e.g., to the MRImode in response to detecting the static magnetic field associated withMRI device 16 and to the magnet mode in response to detecting themagnetic field having a smaller strength, such as that associated with ahandheld magnet. As such, it is desirable that IMD 22 be able toaccurately differentiate between magnetic fields having differentstrengths. The strength of the static magnetic field associated with MRIdevice 16 is typically much larger than the strength of the handheldmagnet or other magnetic fields the patient encounters. As describedabove, MRI device 16 may have a static magnetic field that is largerthan approximately 1.0 Tesla. The strength of the handheld magnet,however, is typically in the millitesla (mT) range. For example, ahandheld magnet may have a strength in the range of approximately 10 mTto 100 mT.

However, the magnetic field sensors of IMD 22 may not always be capableof differentiating the strengths of the magnetic fields. For example,the magnetic field sensors may only be capable of determining that themagnetic field exceeds a threshold (e.g., 1 mT). In this case, IMD 22 isunable to determine whether the magnetic field is from a handheld magnetor a static magnetic field associated with MRI device 16. One techniqueto differentiate the magnetic field of a handheld magnet from themagnetic field associated with MRI device 16 is to monitor for a changein a characteristic of the EGM due to the MHD effect. Because thehandheld magnet generates a magnetic field having a small strength(e.g., 10-100 mT) and small area (e.g., only in the vicinity of IMD 22),there is little, if any, change to the EGM due to the MHD effect.However, the magnetic field generated by MRI device 16 is much larger instrength (e.g., greater than 1.0 T) and applied to a much larger area ofpatient 12 (e.g., an entire section of the body), thereby creating anoticeable change to the EGM due to the MHD effect. As such, monitoringfor a change in the EGM due the MHD effect alone or in combination witha magnetic field sensor or other sensor, provides an effective way todifferentiate magnetic fields of varying strengths. In fact, in someinstances, IMD 22 may be capable of distinguishing between various typesof MRI devices 16 based on the magnitude of the change to the EGM causedby the MHD effect.

IMD 22 may transition back to the normal operating mode after the MRIprocedure or communication session has ended. IMD 22 may automaticallyrevert to the normal operating mode in response to no longer detectingthe magnetic field, whether it was generated by a handheld magnet, MRIdevice 16 or other source, after expiration of a timer or a combinationthereof. Alternatively, IMD 22 may be manually programmed into thenormal operating mode via a command received from an external device 34.

IMD 22 may communicate with external device 34 to exchange data withexternal device 34. External device 34 may, for example, communicatewith IMD 22 to provide the command to transition to the normal operatingmode. As another example, IMD 22 may receive one more operatingparameters for operation of IMD 22 from external device 34. Theoperating parameters may be associated with the MRI mode of operationthat is utilized in response to detecting the static magnetic fieldassociated with MRI device 16. IMD 22 may also transmit sensedphysiological data, diagnostic determinations made based on the sensedphysiological data, IMD performance data and/or IMD integrity data toexternal device 34. IMD 22 and external device 34 may communicate viawireless communication using any techniques known in the art. Examplesof communication techniques may include, for example, low frequency orRF telemetry, although other techniques are also contemplated.

External device 34 may, in some instances, include a telemetry head thatextends from external device 34 and is placed in close proximity to theimplant site of IMD 22 to initiate and perform communication with IMD22. In one example, the telemetry head may include a telemetry headmagnet that generates a magnetic field (“telemetry head field”) and anantenna (not shown) that transmits and receives communications with IMD22. As such, the handheld magnet may be the telemetry head magnet insome examples.

The configuration of implantable medical system 20 illustrated in FIG. 2is merely an example. In other examples, implantable medical system 20may include more or fewer leads extending from IMD 22. For example, IMD22 may be coupled to three leads, e.g., a third lead implanted within aleft ventricle of the heart of the patient. In another example, IMD 22may be coupled to a single lead that is implanted within either anatrium or ventricle of the heart of the patient. As such, IMD 22 may beused for single chamber or multi-chamber cardiac rhythm managementtherapy.

In addition to more or fewer leads, each of the leads may include moreor fewer electrodes. In instances in which IMD 22 is used for therapyother than pacing, e.g., defibrillation or cardioversion, the leads mayinclude elongated electrodes, which may, in some instances, take theform of a coil. IMD 22 may deliver defibrillation or cardioversionshocks to the heart via any combination of the elongated electrodes andhousing electrode. As another example, medical system 20 may includeleads with a plurality of ring electrodes, e.g., as used in someimplantable neurostimulators, without a tip electrode or with one of thering electrodes functioning as the “tip electrode.”

FIG. 3 is a functional block diagram of an example configuration ofelectronic components of IMD 22. IMD 22 includes a control module 40,sensing module 42, therapy module 44, magnetic field sensor 46,communication module 48 and memory 49. The electronic components mayreceive power from a power source (not shown in FIG. 3). In otherexamples, IMD 22 may include more or fewer electronic components. Inaddition, any of the described modules or components may be implementedtogether on a common hardware component or separately as discrete butinteroperable hardware or software components. Depiction of differentfeatures as modules or components is intended to highlight differentfunctional aspects and does not necessarily imply that such modules orunits must be realized by separate hardware or software components.Rather, functionality associated with one or more modules may beperformed by separate hardware or software components, or integratedwithin common or separate hardware or software components.

Memory 49 may include computer-readable instructions that, whenexecuted, cause IMD 22 and/or control module 40 to perform variousfunctions attributed to IMD 22 and control module 40 in this disclosure.In other words, memory 49 includes computer-readable instructions thatcontrol operation of IMD 22. Memory 49 may, for example, store operatingparameters for any of a number of operating modes, including at leastthe normal mode, the magnet mode and one or more MRI modes. Memory 49may include any volatile, non-volatile, magnetic, optical, or electricalmedia, such as a random access memory (RAM), read-only memory (ROM),non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or any other digital media, or combinationthereof.

Control module 40 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated circuitry, including analog circuitry,digital circuitry, or logic circuitry. In some examples, control module40 may include multiple components, such as any combination of one ormore microprocessors, one or more controllers, one or more DSPs, one ormore ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry. The functions attributed to control module40 herein may be embodied as software, firmware, hardware or anycombination thereof.

Control module 40 may communicate with sensing module 42 and therapymodule 44 to operate IMD in the selected operating mode. Sensing module42 and therapy module 44 are electrically coupled to some or all ofelectrodes 28 a,b and 30 a,b via the conductors of leads 24 a,b. Sensingmodule 42 is configured to obtain signals from leads 24 a,b. Controlmodule 40 may process the signals from leads 24 a,b to monitorelectrical activity of the heart of patient 12. Control module 40 may,for example, generate EGM waveforms based on the signals received fromsensing module 42. Control module 40 may also generate marker channeldata based on the detected cardiac activity. For example, marker channeldata may include data that indicates the occurrence and timing ofsensing, diagnosis, and therapy events associated with patient 12 and/orIMD 22.

Control module 40 may store EGM waveforms and marker channel data inmemory 49. Control module 40 may analyze the EGM waveforms and/or markerchannel data to detect cardiac events (e.g., tachyarrhythmias). Controlmodule 40 may also analyze the EGM waveforms to detect changes in thecharacteristics of the EGM indicative of the presence of a strongmagnetic field as will be described in further detail herein. Controlmodule 40 may also later retrieve stored EGMs from memory 49, e.g., upona request from external device 34 received via communication module 48.In further examples, sensing module 42 is coupled to one or more sensorsthat are not included on leads 24 a,b, e.g., via a wired or wirelesscoupling. Such sensors may include pressure sensors, accelerometers,flow sensors, blood chemistry sensors, activity sensors or other typesof physiological sensors. One such sensor is magnetic field sensor 46within IMD 22. Signals monitored by sensing module 42 may be stored inmemory 49.

Therapy module 44 is configured to generate and deliver electricalstimulation therapy to the heart. Control module 40 may control therapymodule 44 to deliver electrical stimulation therapy to the heartaccording to one or more therapy programs, which may be stored in memory49. Control module 40 may, in some instances, control therapy module 44to deliver therapy to patient 12 as a function of the signals sensed bysensing module 42. For example, control module 40 may control therapymodule 44 to trigger and/or inhibit pacing pulses to the heart as afunction of the sensed signals received from sensing module 42. In otherinstances, control module 40 may control therapy module 44 to delivertherapy to patient 12 without regard to signals sensed by sensing module42, such as in an asynchronous pacing mode.

Therapy module 44 may, under the control of control module 40, also beconfigured to generate and deliver cardioversion and defibrillationtherapy to the heart. For example, in the event that control module 40detects an atrial or ventricular tachyarrhythmia, control module 40 mayload an ATP regimen from memory 49, and control therapy module 44 toimplement the ATP regimen. Therapy module 44 may also include a highvoltage charge circuit and a high voltage output circuit that generatehigh voltage shocks to defibrillate the heart.

Communication module 48 includes any suitable hardware, firmware,software or any combination thereof for communicating with anotherdevice, such as external device 34 and/or a patient monitor, e.g., bywireless telemetry. For example, communication module 48 may includeappropriate modulation, demodulation, frequency conversion, filtering,and amplifier components for transmission and reception of data. Underthe control of control module 40, communication module 48 may receivedownlink telemetry from and send uplink telemetry to external device 34with the aid of an antenna (not shown) in IMD 22. Control module 40 mayprovide the data to be uplinked to external device 34 and the controlsignals for a telemetry circuitry within communication module 48, e.g.,via an address/data bus.

IMD 22 may transition from operation in a normal mode to another mode,e.g., MRI mode or magnet mode, in the presence of a magnetic field. Inone example, control module 40 may detect a change in the characteristicof the EGM due to the MHD effect and transition operation of IMD 22 tothe MRI mode in response to the detection. As described above, largestatic magnetic fields, such as those produced by MRI device 16, mayinteract with the blood of patient 12 as it flows through the magneticfield to produce a voltage. The magnitude of the induced voltage (V_(I))may be determined in accordance with the equation:

V _(I) =B*v*d sin θ,  (1)

where B is the strength of the magnetic field, v is the velocity of flowof the blood, d is the diameter of the vessel through which the blood isflowing, and θ is the angle between the direction of the magnetic fieldand the direction of blood flow.

The voltages produced by the MHD effect may result in a change in acharacteristic of an EGM sensed by IMD 22. For example, a voltage may beproduced as the blood flows through the aortic arch upon being ejectedfrom the left ventricle following electrical activation of theventricle. Therefore, the MHD effect is observed subsequent to theR-wave or QRS complex. As such, the voltages produced by the MHD effectmay result in a change in the S-T segment and/or the T-wave. In oneinstance, the voltage produced by the MHD effect may result in anincrease in the amplitude of the T-wave, a change in a duration of theT-wave, a change in a frequency component of the T-wave, a change inmorphology of the T-wave or another change in the T-wave. As such,control module 40 may monitor for such changes in the T-wave, e.g., bycomparing T-wave amplitude changes to a threshold or performing templatematching or wavelet analysis to detect morphology changes. In oneexample, control module 40 may determine the existence of the MHD effectwhen an amplitude of the T-wave changes by at least 50%. In anotherexample, control module 40 may determine the existence of the MHD effectwhen the T-wave of the sensed EGM signal is compared with a T-wave of atemplate EGM signal (collected in the absence of a magnetic field) andhas a correlation coefficient that is less than 0.7.

In another example, the voltage produced by the MHD effect may result ina decrease in a duration or length of the S-T segment, a change in afrequency component of the S-T segment, a change in the morphology ofthe S-T segment or other change in the S-T segment. In this case,control module 40 may monitor for such changes in the S-T segment, e.g.,by comparing S-T segment lengths to a threshold or performing templatematching or wavelet analysis to detect a shape change in the S-Tsegment. In some examples, control module 40 may monitor for oneparticular one of these changes, all of these changes or any combinationof these EGM changes that are indicative of the presence of a strongmagnetic field. In one example, control module 40 may determine theexistence of the MHD effect when a duration of the S-T segment changesby at least 50%. In another example, control module 40 may determine theexistence of the MHD effect when the S-T segment of the sensed EGMsignal is compared with an S-T segment of a template EGM signal(collected in the absence of a magnetic field) and has a correlationcoefficient that is less than 0.7.

As described above, the voltage produced by the MHD effect typicallyoccurs after the ventricular contraction and therefore should not have alarge effect on the P-wave, R-wave or QRS complex of the EGM. As such,control module 40 may monitor for instances in which the change in theS-T segment and/or the T-wave segment of the EGM is accompanied bylittle or no change in the associated P-wave, R-wave or QRS complex. Ifthe change in the S-T segment or T-wave segment is accompanied by achange in the P-wave, R-wave or QRS complex, the change may be due toother causes and not necessarily the MHD effect from a strong magneticfield. Control module 40 may therefore monitor for changes in theamplitude and morphology of the P-wave, R-wave or QRS complex and detectexistence of a strong magnetic field when a change in the S-T segment orthe T-wave segment exceeds the respective threshold and the change inP-wave, R-wave or QRS complex is less than a corresponding threshold.Alternatively, control module 40 may monitor for a change in the EGM bymonitoring for a change in the ratio of P-wave/T-wave amplitudes or aratio of R-wave/T-wave amplitudes.

In response to detecting a change in a characteristic of the EGMindicative of the presence of a strong magnetic field, control module 40may transition to operation of IMD 22 in the MRI mode. In this manner,the change in the characteristic of the EGM due to the MHD effect may beused to detect the MRI device and transition to the MRI mode.

As indicated by equation (1) above, there is a linear relationshipbetween the induced voltage and the strength of the magnetic field,i.e., the voltage induced by the magnetic field will increase linearlywith the strength of the magnetic field. Therefore, the change in theEGM signal, e.g., amplitude of the T-wave, may increase linearly withthe strength of the magnetic field. For example, an MRI device having astatic magnetic field of 1.5 Tesla may result in a change in theamplitude of T-wave that is smaller than the change in the amplitude ofthe T-wave caused by an MRI device having a static magnetic field of 3.0Tesla. Control module 40 may therefore differentiate between differenttypes of MRI devices based on the amount of change or magnitude of thechange in the EGM signal. Control module 40 may transition operation ofIMD 22 to an MRI mode corresponding with a type of MRI device 16. Inother words, IMD 22 may have multiple MRI modes that each correspondwith a particular type of MRI device and have different settings, e.g.,different filter settings to filter out different frequencies. In someinstances, implantable medical system 20 may only be approved for usewith particular MRI devices. For example, implantable medical system 20may be approved for use in a 1.5 Tesla MRI device. Thus, if a 3.0 T MRIdevice is detected, IMD 22 may generate an audible, visible or tactilealert notifying the patient or a telemetry alert to notify a technicianor physician that IMD 22 is not approved for use in a 3.0 T MRI device.

In some instances, control module 40 may monitor for the MHD effect inconjunction with other criteria. In one embodiment, control module 40may obtain the output of magnetic field sensor 46 and adjust operationof IMD 22 based on the output of magnetic field sensor 46 and thechanges in the EGM signal. This may be useful when magnetic field sensor46 is unable to differentiate between magnetic field strengths. Forexample, magnetic field sensor 46 may capable of determining whether amagnetic field exceeds a threshold, but not be capable ofdifferentiating between two magnetic fields of different strength thatboth exceed the threshold. In other words, magnetic field sensor 46 maybe a single threshold sensor that indicates whether a magnetic fieldexceeds a threshold (e.g., 1 mT). In this case, however, the singlethreshold sensor is not capable of differentiating between a 10-100 mTmagnetic field (which may be the strength of a handheld magnet) and a3.0 T magnetic field (which may be the strength of the static field ofan MRI device 16). IMD 22 may therefore be unable to determine whetherthe magnetic field is from a handheld magnet (in which case the controlprocessor should transition to the magnet mode) or is from a staticmagnetic field associated with MRI device 16 (in which case the controlprocessor should transition to the MRI mode).

Control module 40 may analyze the EGM signal in addition to the outputof the magnetic field sensor to differentiate between magnetic fieldsthat exceed the threshold of magnetic field sensor 46. As describedabove, the magnetic field generated by handheld magnet does not producemuch, if any, voltage caused by the MHD effect. To the extent a voltageis produced due to the MHD effect during application of handheld magnet,the induced voltage is small enough and far enough away from sensingelectrodes of leads 24 to not change the characteristics of the EGM. Onthe other hand, the static magnetic field generated by MRI device 16 islarger in both strength and area of application. The voltages induced bythe MHD effect from the static magnetic field of MRI device 16 are muchlarger than those induced by handheld magnet and may result in changesto the EGM. As such, control module 40 may determine that the magneticfield is associated with an MRI device 16 when the strength of themagnetic field exceeds the threshold of magnetic field sensor 46 andcontrol module 40 detects the changes in the EGM corresponding with theMHD effect (e.g., changes in the S-T segment or T-wave in conjunctionwith relatively no change in the P-wave, R-wave or QRS complex). In thiscase, control module 40 may transition operation of IMD 22 to the MRImode. Control module 40 may determine that the magnetic field isassociated with handheld magnet when the strength of the magnetic fieldexceeds the threshold of magnetic field sensor 46 and control module 40does not detect changes in the EGM corresponding with the MHD effect. Inthis case, control module 40 may transition operation of IMD 22 to themagnet mode. As such, monitoring for a change in the EGM due the MHDeffect alone or in combination with a magnetic field sensor or othersensor, provides an effective way to differentiate magnetic fields ofvarying strengths. In one example, control module 40 may begin analyzingthe EGM signal for the MDH effect in response to the magnetic fieldsensor 46 detecting presence of a magnetic field above the threshold,thereby reserving processing resources and power. In other examples,control module 40 may continuously be monitoring for changes in the EGMsignal whether or not magnetic field sensor 46 detects presence of amagnetic field above the threshold.

As indicated in equation (1) above, the induced voltage is also relatedto the angle between the direction of the magnetic field and thedirection of flow, i.e., the closer the blood flow is to beingperpendicular to the direction of the magnetic field, the larger theinduced voltage. A large induced voltage occurs when the blood isflowing through the aortic arch of the heart of patient 12, which isboth larger in diameter than most of the other vasculature of the heartand because the aortic arch is more perpendicular to the direction ofthe magnetic field than other vasculature of the heart. Sensing thevoltage induced by the MHD effect on the blood flowing through theaortic arch may be better sensed using certain sensing vectors. Forexample, a unipolar sensing vector may better sense the induced voltagethan a bipolar sensing vector. As another example, sensing vectorbetween an SVC coil and an RV coil of a defibrillation lead may bebetter than a bipolar sensing vector. To this end, control module 40 mayreconfigure a sensing vector that is analyzed to monitor for a change ina characteristic of the EGM signal in response to magnetic field sensor46 detecting a magnetic field that exceeds the threshold of magneticfield sensor 46. In one example, control module 40 may change thesensing vector from a bipolar sensing vector (e.g., tip-to-ring) to aunipolar sensing vector (e.g., tip-to-can, ring-to-can or coil-to-can)or to a SVC-to-RV sensing vector to monitor for the change to the EGMsignal from the MHD effect.

After the MRI procedure is complete, control module 40 may transitionoperation of IMD 22 back to the normal mode of operation, e.g., turnhigh voltage therapy back on and/or have pacing that is triggered and/orinhibited as a function of sensed signals. Control module 40 mayautomatically revert to the normal mode of operation in response tomagnetic field sensor 46 no longer detecting presence of the magneticfield, control module 40 no longer detecting the MHD effect on the EGMsignal, after expiration of a timer or a combination thereof.Alternatively, control module 40 may transition operation of IMD 22 backto the normal mode in response to a command received from externaldevice 34, such as programming device.

FIG. 4 is a flow diagram illustrating example operation of animplantable medical device to detect presence of a magnetic field basedon a change in a characteristic of an EGM signal. Control module 40 ofIMD 22 analyzes a sensed EGM signal (52). Control module 40 determineswhether a characteristic of the EGM signal is indicative of the presenceof a strong magnetic field (54). As described above, the strong magneticfield may induce a voltage due to the MHD effect that may change acharacteristic of the EGM signal. The voltage produced by the MHD effectmay result in a change in the S-T segment and/or the T-wave of the EGMsignal. In one example, the voltage produced by the MHD effect mayresult in an increase in the amplitude of the T-wave, a change inmorphology of the T-wave or another change in the T-wave. In anotherexample, the voltage produced by the MHD effect may result in a decreasein the length of the S-T segment, a change in the shape of the S-Tsegment or other change in the S-T segment. In some instances, thechange to the S-T segment or T-wave of the EGM must be accompanied byrelatively little or no change in the P-wave, R-wave or QRS complex ofthe EGM.

In response to detecting a change in a characteristic of the EGM (“YES”branch of block 54), control module 40 may transition operation of IMD22 to an MRI mode (56). In this manner, the change in the characteristicof the EGM due to the MHD effect may be used to detect the MRI deviceand transition to the MRI mode. In response to not detecting a change ina characteristic of the EGM signal (“NO” branch of block 54), controlmodule continues to analyze subsequent EGM signals.

FIG. 5 is a flow diagram illustrating example operation of animplantable medical device distinguishing between different types ofmagnetic fields based on a change in a characteristic of an EGM signal.Control module 40 obtains output of magnetic field sensor 46 (60).Control module 40 determines whether a magnetic field is detected basedon the output of the magnetic field sensor 46 (62). Magnetic fieldsensor 46 may, for example, output a first value when a magnetic fieldwithin the vicinity of IMD 22 does not exceed a threshold (e.g., 1 mT)and output a second value when a magnetic field within the vicinity ofIMD 22 exceeds the threshold. Control module 40 may detect a magneticfield when the output of magnetic field sensor 46 is the second value.

When control module 40 does not detect a magnetic field (“NO” branch ofblock 62), control module 40 continues to obtain the output of magneticfield sensor 46. When control module 40 does detect a magnetic field(“YES” branch of block 62), control module 40 analyzes a sensed EGMsignal (64). Control module 40 determines whether a characteristic ofthe EGM signal indicative of the presence of a strong magnetic fieldexists (66). As described above, control module 40 may analyze the EGMto monitor for a change in the S-T segment and/or the T-wave of the EGMsignal indicative of the MHD effect caused by a strong magnetic field.In one example, the change to the S-T segment or T-wave of the EGM mustbe accompanied by relatively little or no change in the correspondingP-wave, R-wave or QRS complex of the EGM.

In response to not detecting a change in a characteristic of the EGMsignal (“NO” branch of block 66), control module 40 transitionsoperation of IMD 22 to a magnet mode (68). In response to detecting achange in a characteristic of the EGM (“YES” branch of block 66),control module 40 transitions operation of IMD 22 to an MRI mode (70).In this manner, the change in the characteristic of the EGM due to theMHD effect may be used to differentiate between magnetic fields producedby different sources, e.g., differentiate a magnetic field produced by ahandheld magnet (which may be between 10-100 mT) from a magnetic fieldproduced by an MRI device 16 (which may be greater than 1 T).

FIG. 6 is a flow diagram illustrating another example operation of animplantable medical device distinguishing between different types ofmagnetic fields based on a change in a characteristic of an EGM signal.Control module 40 obtains output of magnetic field sensor 46 (70).Control module 40 determines whether a magnetic field is detected basedon the output of the magnetic field sensor 46 (72). Magnetic fieldsensor 46 may, for example, output a first value when a magnetic fieldwithin the vicinity of IMD 22 does not exceed a threshold (e.g., 1 mT)and output a second value when a magnetic field within the vicinity ofIMD 22 exceeds the threshold. Control module 40 may detect a magneticfield when the output of magnetic field sensor 46 is the second value.

When control module 40 does not detect a magnetic field (“NO” branch ofblock 72), control module 40 continues to obtain the output of magneticfield sensor 46. When control module 40 does detect a magnetic field(“YES” branch of block 72), control module 40 switches a sensing vectorof IMD 22 (74) Sensing the voltage induced by the MHD effect on theblood flowing through specific heart chambers or blood vessels (e.g.,the aortic arch) may be better sensed using certain sensing vectors. Forexample, a unipolar sensing vector may better sense the induced voltagethan a bipolar sensing vector. As another example, a sensing vectorbetween an SVC coil and an RV coil of a defibrillation lead may bebetter than a bipolar sensing vector. In one example, control module 40may change the sensing vector from a bipolar sensing vector (e.g.,tip-to-ring or tip-to-RV coil) to a unipolar sensing vector (e.g.,tip-to-can, ring-to-can, coil-to-can or tip-to-SVC coil) or to a SVCcoil-to-RV coil sensing vector to monitor for the change to the EGMsignal from the MHD effect. In this manner, the sensing vector may beoptimized for sensing an EGM with a more prominent change in thecharacteristic of the EGM due to the MHD effect.

Control module 40 analyzes the EGM signal sensed using the adjustedsensing vector (76). Control module 40 determines whether acharacteristic of the EGM signal indicative of the presence of a strongmagnetic field exists (78). As described above, control module 40 mayanalyze the EGM to monitor for a change in the S-T segment and/or theT-wave of the EGM signal indicative of the MHD effect caused by a strongmagnetic field. In one example, the change to the S-T segment or T-waveof the EGM must be accompanied by relatively little or no change in thecorresponding P-wave, R-wave or QRS complex of the EGM.

In response to not detecting a change in a characteristic of the EGMsignal (“NO” branch of block 78), control module 40 transitionsoperation of IMD 22 to a magnet mode (80). In response to detecting achange in a characteristic of the EGM (“YES” branch of block 78),control module 40 transitions operation of IMD 22 to an MRI mode (82).Thus, the change in the characteristic of the EGM due to the MHD effectmay be used to differentiate between magnetic fields produced bydifferent sources.

FIG. 7 is a flow diagram illustrating another example operation of animplantable medical device distinguishing between different types ofmagnetic fields based on a change in a characteristic of an EGM signal.Control module 40 obtains output of magnetic field sensor 46 (90).Control module 40 determines whether a magnetic field is detected basedon the output of the magnetic field sensor 46 (92). Magnetic fieldsensor 46 may, for example, output a first value when a magnetic fieldwithin the vicinity of IMD 22 does not exceed a threshold (e.g., 1 mT)and output a second value when a magnetic field within the vicinity ofIMD 22 exceeds the threshold. Control module 40 may detect a magneticfield when the output of magnetic field sensor 46 is the second value.

When control module 40 does not detect a magnetic field (“NO” branch ofblock 92), control module 40 continues to obtain the output of magneticfield sensor 46. When control module 40 does detect a magnetic field(“YES” branch of block 92), control module 40 analyzes a sensed EGMsignal (94). Control module 40 determines whether a characteristic ofthe EGM signal indicative of the presence of a strong magnetic fieldexists (96). As described above, control module 40 may analyze the EGMto monitor for a change in the S-T segment and/or the T-wave of the EGMsignal indicative of the MHD effect caused by a strong magnetic field.In one example, the change to the S-T segment or T-wave of the EGM mustbe accompanied by relatively little or no change in the correspondingP-wave, R-wave or QRS complex of the EGM.

In response to not detecting a change in a characteristic of the EGMsignal (“NO” branch of block 96), control module 40 transitionsoperation of IMD 22 to a magnet mode (98). In response to detecting achange in a characteristic of the EGM (“YES” branch of block 96),control module 40 determines a magnitude of the change of thecharacteristic of the EGM signal (100). As indicated by equation (1)above, there is a linear relationship between the induced voltage andthe strength of the magnetic field, i.e., the voltage induced by themagnetic field will increase linearly with the strength of the magneticfield. Therefore, the change in the EGM signal, e.g., amplitude of theT-wave, may increase linearly with the strength of the magnetic field.For example, an MRI device having a static magnetic field of 1.5 Teslamay result in a change in the amplitude of T-wave that is smaller thanthe change in the amplitude of the T-wave caused by an MRI device havinga static magnetic field of 3.0 Tesla. Control module 40 may thereforedifferentiate between different types of MRI devices based on themagnitude of the change in the EGM signal.

Control module 40 selects an MRI mode based on the magnitude of thechange in the EGM signal (102). In other words, IMD 22 may have multipleMRI modes that each correspond with a particular type of MRI device.Control module 40 may select the MRI mode that corresponds with themagnitude of the change in the EGM signal, e.g., based on an amount thatthe amplitude of the T-wave increased. Control module 40 transitionsoperation of IMD 22 to the selected MRI mode (104). In this manner, thechange in the characteristic of the EGM due to the MHD effect may beused to differentiate between magnetic fields produced by more than twodifferent sources.

The techniques described herein may be applicable to other therapysystems. For example, the techniques described herein may be applicableto systems including an IMD that delivers electrical stimulation therapyto other muscles, nerves or organs of patient 12. As another example,the techniques described herein may be applicable to systems includingan implantable drug delivery or infusion device or an IMD including adrug delivery or infusion module. Other combinations of implantabledevices will be obvious to one of skill in the art, and fall within thescope of this disclosure.

The techniques described in this disclosure, including those attributedto IMD 22, may be implemented, at least in part, in hardware, software,firmware or any combination thereof. For example, various aspects of thetechniques may be implemented within one or more processors, includingone or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalentintegrated or discrete logic circuitry, as well as any combinations ofsuch components, embodied in programmers, such as physician or patientprogrammers, stimulators, or other devices. The term “processor” maygenerally refer to any of the foregoing circuitry, alone or incombination with other circuitry, or any other equivalent circuitry.

Such hardware, software, or firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, or flash memory, magnetic data storage media, optical datastorage media, or the like. The instructions may be executed to supportone or more aspects of the functionality described in this disclosure.

Various examples have been described. These and other examples arewithin the scope of the following claims.

1. A method comprising: detecting presence of a magnetic field based onthe output of a magnetic field sensor; monitoring an electrogram (EGM)measured by an implantable medical device in response to detecting thepresence of the magnetic field; identifying a change in a characteristicof the EGM indicative of the presence of the magnetic field; andadjusting operation of the implantable medical device based on theoutput of the magnetic field sensor and the change in the characteristicof the EGM.
 2. The method of claim 1, wherein adjusting operation of theimplantable medical device comprises transitioning from a current modeto an MRI mode in response to detecting the presence of the magneticfield based on the output of the magnetic field sensor and identifyingthe change in the characteristic of the EGM, wherein the MRI mode isdifferent than the current mode.
 3. The method of claim 2, whereinadjusting operation of the implantable medical device comprisestransitioning from a current mode to a magnet mode in response todetecting the presence of the magnetic field based on the output of themagnetic field sensor and not identifying the change in thecharacteristic of the EGM, wherein the magnet mode is different than theMRI mode and the current mode.
 4. The method of claim 2, whereinadjusting operation of the implantable medical device from the currentmode to an MRI mode comprises: determining a magnitude of the change inthe characteristic of the EGM; selecting one of a plurality of possibleMRI modes based on a magnitude of the change in the characteristic ofthe EGM; and transitioning operation of the implantable medical deviceto the selected MRI mode.
 5. The method of claim 1, further comprising:switching a sensing vector used to sense the EGM in response todetecting presence of the magnetic field based on the output of themagnetic field sensor; and monitoring for the change in thecharacteristic of the EGM using EGM data sensed using the switchedsensing vector.
 6. The method of claim 6, wherein switching to adifferent sensing vector comprises switching to one of a unipolarsensing vector and an SVC coil to RV coil sensing vector in response todetecting presence of the magnetic field based on the output of amagnetic field sensor.
 7. The method of claim 1, wherein identifying thechange in the characteristic of the EGM indicative of the presence of amagnetic field comprises identifying a change in a T-wave of the EGM. 8.The method of claim 7, wherein identifying the change in the T-wavecomprises one of identifying a change in an amplitude of the T-wave, achange in a duration of the T-wave, a change in a frequency component ofthe T-wave, and a change in a morphology of the T-wave.
 9. The method ofclaim 1, wherein identifying a change in a characteristic of the EGMindicative of the presence of a magnetic field comprises identifying achange in a T-wave of the EGM in which at least one of the correspondingP-wave, R-wave and QRS complex is substantially unchanged.
 10. Themethod of claim 1, wherein identifying the change in the characteristicof the EGM comprises identifying a change in an S-T segment of the EGM.11. The method of claim 10, wherein identifying the change in the S-Tsegment of the EGM further comprises identifying the change in aduration of the S-T segment, a change in a frequency component of theS-T segment, and a change in a morphology of the S-T segment.
 12. Animplantable medical system comprising: at least one implantable medicallead that includes at least one electrode; and an implantable medicaldevice connected to the medical lead, the implantable medical devicecomprising: a magnetic field sensor; and a control module configured todetect presence of a magnetic field based on the output of the magneticfield sensor, monitor an electrogram (EGM) measured by an implantablemedical device in response to detecting the presence of the magneticfield, identify a change in a characteristic of the EGM indicative ofthe presence of the magnetic field, and adjust operation of theimplantable medical device based on the output of the magnetic fieldsensor and the change in the characteristic of the EGM.
 13. The systemof claim 12, wherein the control module transitions operation of theimplantable medical device from a current mode to an MRI mode inresponse to detecting the magnetic field based on the output of themagnetic field sensor and identifying the change in the characteristicof the EGM, wherein the MRI mode is different than the current mode. 14.The system of claim 13, wherein the control module transitions operationof the implantable medical device from the current mode to a magnet modein response to detecting the magnetic field based on the output of themagnetic field sensor and not identifying the change in thecharacteristic of the EGM, wherein the magnet mode is different than theMRI mode and the current mode.
 15. The system of claim 13, wherein thecontrol module determines a magnitude of the change in thecharacteristic of the EGM, selects one of a plurality of possible MRImodes based on the magnitude of the change in the characteristic of theEGM and transitions operation of the implantable medical device to theselected MRI mode.
 16. The system of claim 12, wherein the implantablemedical lead includes at least two electrodes, and the control moduleswitches a sensing vector used to sense the EGM in response detectingpresence of the magnetic field based on the output of a magnetic fieldsensor and monitors for the change in the characteristic of the EGMusing EGM data sensed using the switched sensing vector.
 17. The systemof claim 16, wherein the control module switches to one of a unipolarsensing vector and an SVC coil to RV coil sensing vector in responsedetecting presence of the magnetic field based on the output of amagnetic field sensor.
 18. The system of claim 12, wherein the controlmodule identifies a change in a T-wave of the EGM indicative of thepresence of a magnetic field.
 19. The system of claim 18, wherein thecontrol unit identifies one of a change in an amplitude of the T-wave ofthe EGM, a change in a duration of the T-wave, a change in a frequencycomponent of the T-wave, and a change in a morphology of the T-wave ofthe EGM indicative of the presence of a magnetic field.
 20. The systemof claim 12, wherein the control unit identifies a change in a T-wave ofthe EGM that corresponds with one of a P-wave, R-wave and QRS complexthat is substantially unchanged.
 21. The system of claim 12, wherein thecontrol module identifies a change in an S-T segment of the EGMindicative of the presence of a magnetic field.
 22. The system of claim21, wherein the control module identifies a change in at least one of aduration of the S-T segment, a change in a frequency component of theS-T segment, and a change in a morphology of the S-T segment indicativeof the presence of a magnetic field.
 23. A computer-readable mediumcomprising instructions that, when executed, cause an implantablemedical device to: detect presence of a magnetic field based on theoutput of a magnetic field sensor; monitor an electrogram (EGM) measuredby an implantable medical device in response to detecting the presenceof the magnetic field; identify a change in a characteristic of the EGMindicative of the presence of the magnetic field; and adjust operationof the implantable medical device based on the output of the magneticfield sensor and the change in the characteristic of the EGM.